The present invention relates to telecommunications apparatus, methods, systems and apparatus for transmitting data to and/or receiving data from mobile terminals in a wireless communications system. Example embodiments of the present technique can provide a facility for varying the level of power in selected transmissions.
There is an approximate relationship between coverage within a wireless communications system and the power with which signals are transmitted from infrastructure equipment such as base stations (e.g. eNodeBs) and/or network controllers (e.g. RNCs, eNodeBs) to user equipment (UE—i.e. wireless communications devices).
A distance between infrastructure equipment and user equipment (UE) is the main factor in determining the power in signals received from the infrastructure equipment at the UE. The further apart the UE and infrastructure equipment lie, the greater the attenuation a signal will experience until the point when the attenuated signal has a power level of the same order as the ambient noise.
A location of a UE can also determine whether the transmission power is adequate. UEs positioned indoors or underground experience significant attenuation: being within a predetermined radial distance of infrastructure equipment may be a necessary condition but is not a sufficient one.
An anticipated widespread deployment of third and fourth generation cellular networks has led to the parallel development of a class of devices and applications which, rather than taking advantage of the high data rates available, instead take advantage of the robust radio interface and increasing ubiquity of the coverage area. This parallel class of devices and applications includes MTC devices and so-called machine to machine (M2M) applications, wherein semi-autonomous or autonomous wireless communication devices typically communicate small amounts of data on a relatively infrequent basis.
Unlike a conventional third or fourth generation terminal device such as a smartphone, an MTC-type terminal is preferably relatively simple and inexpensive: in addition MTC-devices are often deployed in situations that do not afford easy access for direct maintenance or replacement—reliable and efficient operation can be crucial. Furthermore, while the type of functions performed by the MTC-type terminal (e.g. collecting and reporting back data) do not require particularly complex processing to perform, third and fourth generation mobile telecommunication networks typically employ advanced data modulation techniques (such as 16QAM or 64QAM) on the radio interface which can require more complex and expensive radio transceivers to implement.
A “virtual carrier” tailored to low capability terminals such as MTC devices is thus provided within the transmission resources of a conventional OFDM type downlink carrier (i.e. a “host carrier”). Unlike data transmitted on a conventional OFDM type downlink carrier, data transmitted on the virtual carrier can be received and decoded without needing to process the full bandwidth of the downlink host OFDM carrier, for at least some part of a subframe. Accordingly, data transmitted on the virtual carrier can be received and decoded using a reduced complexity receiver unit.
As noted above, the nature of MTC devices can lead to their deployment in locations where radial distance to the infrastructure equipment is not the only significant factor in attenuation of signals. To improve coverage for such devices, it would be desirable to provide signalling at a higher transmission power. Coverage can then be extended by ensuring that data is transmitted at a sufficiently high power that the MTC device can receive the signal.
To permit channel estimation LTE however relies upon the transmission power of certain symbols, known as reference or pilot symbols. Channel estimation refers to the facility for measuring channel characteristics (such as the complex gain) at certain, predetermined, positions in a radio frame (i.e. times and/or frequencies) so that the approximate channel characteristics at all positions in the frame can be deduced. Channel estimates are in turn used to equalize the effects of noise on all channels.
As the transmission power of the reference signals is one of the channel characteristics measured in channel estimation, these reference signals cannot be transmitted at different powers without disrupting the channel estimation and subsequent equalization functions. For those UEs where greater attenuation is experienced (such as MTC devices installed in cellars), this leads to a situation where the reference symbols are attenuated to such an extent they are inadequate for channel estimation.